Analog-to-digital converter (ADC) is usually applied to the front ends of many signal processing schemes to convert an analog signal to a digital signal. Sigma-delta modulator, one kind of analog-to-digital converter, has being broadly applied in a high resolution data conversion technology; main skill used in the sigma-delta modulator for obtaining high resolution are over-sampling, noise shaping and filtering. Furthermore, the technology has successfully applied to direct current measurement, voice frequency and sound frequency signal processing, integrated service digital network (ISDN) and communication system.
In general, summation circuit, filter, quantizer, and digital-to-analog converter (DAC) are included in the sigma-delta modulator, in which the filter may be implemented by a switched-capacitor integrator for filtering signals to increase a signal-to-noise ratio (SNR) of the sigma-delta modulator. Besides, feedback path in the filter can allow a conjugate zero being generated on a noise transfer function (NTF) spectrum of the sigma-delta modulator, as shown in FIG. 1. The conjugate zero can extend a plain scope of a signal frequency band and further suppress quantified noise in the signal frequency band.
However, when the sigma-delta modulator is applied to a voice signal product, an over-sampling ratio (OSR) is 64 or larger such that a product of a gain and an integral value of the local feedback will be very small, such as 0.002. If it is implemented by the switched-capacitor integrator, the product of the gain and the integral value of the local feedback must be equal to a value obtained by dividing a capacitance value of the feedback capacitor connected onto the feedback route with a capacitance value of an integrating capacitor. But, the capacitance value of the integrating capacitor will become very large under a condition that the feedback capacitor cannot be too small such that the area, the cost and a stray capacitance value at an output end of an operation amplifier increase.